Method for cleaning thin-film forming apparatus

ABSTRACT

This invention is a cleaning method of a film-forming unit that forms a thin film on an object to be processed by supplying a process gas into a reaction chamber containing the object to be processed, the method comprising a purging step of purging an inside of the reaction chamber by supplying into the reaction chamber a nitrogen-including gas that includes nitrogen and that is capable of being activated. The purging step has a step of nitriding a surface of a member in the reaction chamber by activating the nitrogen-including gas.

FIELD OF THE INVENTION

This invention relates to a method for cleaning a film-forming unit, inparticular to a method for cleaning a film-forming unit by removingreaction products stuck in a discharging system such as a dischargingduct in the film-forming unit.

BACKGROUND ART

In some steps for manufacturing semiconductor device, a thin film isformed on an object to be processed such as a semiconductor wafer byconducting a process such as a CVD (Chemical Vapor Deposition) process.For example, a thermal processing unit shown in FIG. 8 is used for sucha film-forming process.

The film-forming process by the thermal processing unit 51 shown in FIG.8 is conducted as follows. At first, a double-tube-type reactive tube 52consisting of an inner tube 52 a and an outer tube 52 b is heated to apredetermined temperature, for example 760° C., by a heater 53. Then, awafer boat 55 containing a plurality of semiconductor wafers 54 isloaded into the reaction tube 52 (the inner tube 52 a). Then, gas in thereaction tube 52 is discharged through a discharging port 56 in order todecompress an inside of the reaction tube 52 to a predeterminedpressure, for example 26.5 Pa (0.2 Torr). After the inside of thereaction tube 52 is decompressed to the predetermined pressure, aprocess gas is supplied from a gas introducing pipe 57 into the innertube 52 a. When the process gas is supplied into the inner tube 52 a,the process gas causes a thermal reaction, so that reaction productsgenerated thereby are deposited on surfaces of the plurality ofsemiconductor wafers 54. Then, a thin film is formed onto each of theplurality of semiconductor wafers 54.

Exhaust gas generated in the film-forming process is discharged throughthe discharging port 56 and a discharging duct 58, outside the thermalprocessing unit 51. A trap or a scrubber, not shown, is provided in thedischarging duct 58 in order to remove reaction products contained inthe exhaust gas.

Herein, the reaction products generated during the film-forming processmay be deposited not only on the surfaces of the semiconductor wafers54, but also on inner surfaces of the thermal processing unit 51, forexample on an inner wall of the inner tube 52 a. If the film-formingprocess is continued with the reaction products sticking to them, thereaction products may peel off to become particles. The particles maystick to the semiconductor wafers 54. Thus, a yield of manufacturedsemiconductor devices may tend to be low.

Thus, in the conventional thermal processing unit, for example, afilm-forming process is conducted only such times that no particles aregenerated. After that, the inside of the thermal processing unit 51 isheated to a predetermined temperature by the heater 53, a mixed gas of afluorine gas and a halogen-including acid gas (cleaning gas) is suppliedinto the heated thermal processing unit 51, and the reaction productsstuck on the inner surfaces of the thermal processing unit 51 such asthe inner wall of reaction tube 52 are removed (dry-etched) (forexample, JP Laid-Open publication No. Hei 3-293726).

However, when the cleaning gas is supplied into the thermal processingunit 51, the fluorine contained in the cleaning gas diffuses into amaterial of the reaction tube 52, for example quartz. Even if a nitrogengas is supplied into the thermal processing unit 51 after that, thefluorine tends not to be discharged outside the thermal processing unit41. In addition, if a film-forming process is conducted under acondition wherein the fluorine has diffused into the quartz forming thereaction tube 52, the fluorine may diffuse (outwardly diffuse) from thereaction tube 52 during the film-forming process. In the case, fluorinedensity in a film formed on a semiconductor wafer 54 may be increased.

In addition, if the fluorine diffuses outward from the reaction tune 52,fluorine impurities (for example, SiF) may be mixed into a film formedon a semiconductor wafer 54. If the fluorine impurities are mixed, ayield of manufactured semiconductor devices may be deteriorated.

In addition, in the conventional thermal processing unit 51, afilm-forming process for depositing the reaction products on thesurfaces of the semiconductor wafers 54 is repeatedly conducted in thereaction tube 52 maintained at a high temperature and a low pressure.Thus, even if the inside of the unit is periodically cleaned, a minuteamount of impurities may be discharged (generated) from the quartz thatis a material forming the reaction tube 52. For example, in the quartzthat is a material forming the reaction tube 52, a minute amount ofmetallic contaminant such as copper is included. Then, the metalliccontaminant may diffuse outward from the reaction tube 52 during afilm-forming process. If the impurities such as the metallic contaminantstick to the semiconductor wafers 54, a yield of manufacturedsemiconductor devices may be deteriorated.

SUMMARY OF THE INVENTION

This invention is intended to solve the above problems effectively. Anobject of this invention is to provide a film-forming unit, a cleaningmethod of the film-forming unit and a film-forming method, wherein itcan be prevented that impurities are mixed into a formed thin film.

In addition, another object of this invention is to provide afilm-forming unit, a cleaning method of the film-forming unit and afilm-forming method, which can inhibit diffusion of impurities such asfluorine, metallic contaminant and so on.

Furthermore, another object of this invention is to provide afilm-forming unit, a cleaning method of the film-forming unit and afilm-forming method, which can low inhibit density of impurities such asfluorine, metallic contaminant and so on.

In order to achieve the above objects, a cleaning method of afilm-forming unit according to this invention is a cleaning method of afilm-forming unit that forms a thin film on an object to be processed bysupplying a process gas into a reaction chamber containing the object tobe processed, the method comprising a purging step of purging an insideof the reaction chamber by supplying into the reaction chamber anitrogen-including gas that includes nitrogen and that is capable ofbeing activated, wherein the purging step has a step of nitriding asurface of a member in the reaction chamber by activating thenitrogen-including gas.

According to the invention, a surface of a member in the reactionchamber, for example a surface of a member forming the reaction chamber,is nitrided by the activated nitrogen-including gas. Thus, it becomesdifficult for impurities to be discharged from the member in thereaction chamber, so that it can be prevented that the impurities aremixed into a formed thin film.

Alternatively, this invention is a cleaning method of a film-formingunit that forms a thin film on an object to be processed by supplying aprocess gas into a reaction chamber containing the object to beprocessed, the method comprising a purging step of purging an inside ofthe reaction chamber by supplying into the reaction chamber anitrogen-including gas that includes nitrogen and that is capable ofbeing activated, wherein the purging step has a step of activating thenitrogen-including gas and causing the activated nitrogen-including gasto react with metallic contaminant contained in a member in the reactionchamber so as to remove the metallic contaminant from the member.

According to the feature, the activated nitrogen-including gas reactswith the metallic contaminant contained in a member in the reactionchamber, for example a member forming the reaction chamber, and thus themetallic contaminant is removed from the member. Therefore, an amount ofmetallic contaminant contained in the member in the reaction chamber maybe reduced, and diffusion of the metallic contaminant during thefilm-forming process may be inhibited. Thus, density of the metalliccontaminant in a formed film may be reduced. In addition, it becomesdifficult for impurities to be mixed into a formed film.

Alternatively, this invention is a cleaning method of a film-formingunit that forms a thin film on an object to be processed by supplying aprocess gas into a reaction chamber containing the object to beprocessed, the method comprising: a deposit-removing step of removing adeposit stuck to an inside of the film-forming unit by supplying intothe reaction chamber a cleaning gas that includes fluorine, and apurging step of purging an inside of the reaction chamber by supplyinginto the reaction chamber a nitrogen-including gas that includesnitrogen and that is capable of being activated, wherein the purgingstep has a step of activating the nitrogen-including gas and causing theactivated nitrogen-including gas to react with the fluorine diffusedinto a member in the reaction chamber during the deposit-removing step,so as to remove the fluorine from the member.

According to the feature, the activated nitrogen-including gas reactswith the fluorine diffused into a member in the reaction chamber, forexample a member forming the reaction chamber, and thus the fluorine isremoved from the member. Therefore, an amount of fluorine diffused intothe member in the reaction chamber may be reduced, and diffusion of thefluorine during the film-forming process may be inhibited. Thus, densityof the fluorine in a formed film may be reduced. In addition, it becomesdifficult for impurities to be mixed into a formed film.

Alternatively, this invention is a cleaning method of a film-formingunit that forms a thin film on an object to be processed by supplying aprocess gas into a reaction chamber containing the object to beprocessed, the method comprising: a deposit-removing step of removing adeposit stuck to an inside of the film-forming unit by supplying intothe reaction chamber a cleaning gas that includes fluorine, and apurging step of purging an inside of the reaction chamber by supplyinginto the reaction chamber a nitrogen-including gas that includesnitrogen and that is capable of being activated, wherein the purgingstep has a step of nitriding a surface of a member in the reactionchamber by activating the nitrogen-including gas.

According to the feature, a surface of a member in the reaction chamber,for example a surface of a member forming the reaction chamber, isnitrided by the activated nitrogen-including gas. Thus, it becomesdifficult for the fluorine to diffuse (be discharged) from the member inthe reaction chamber, so that diffusion of the fluorine during thefilm-forming process may be inhibited. Thus, density of the fluorine ina formed film may be reduced. In addition, it can be inhibited thatimpurities are mixed into a formed film.

The nitrogen-including gas is, for example, ammonia, dinitrogen monoxideor nitric oxide.

For example, during the purging step, the inside of the reaction chamberis maintained at a range of 133 Pa to 53.3 kPa.

For example, during the purging step, the nitrogen-including gas issupplied into the reaction chamber heated to a predetermined temperaturein order to be activated.

Preferably, during the purging step, the inside of the reaction chamberis heated to a range of 600° C. to 1050° C.

For example, the member in the reaction chamber consists of quartz.

For example, the process gas comprises ammonia and a silicon-includinggas, the thin film is a silicon nitride film, and the nitrogen-includinggas is an ammonia gas. In the case, for example, the silicon-includinggas is dichlorosilane, hexachlorosilane, monosilane, disilane,tetrachlorosilane, trichlorosilane, bis(tert-butylamino)silane orhexaethyl(amino)disilane.

In addition, this invention is a film-forming method comprising: acleaning step of cleaning a film-forming unit in accordance with acleaning method of a film-forming unit according to any of the abovefeatures, and a film-forming step of heating the inside of the reactionchamber containing the object to be processed to a predeterminedtemperature, and forming a thin film on the object to be processed bysupplying a process gas into the reaction chamber.

According to the invention, it becomes difficult for impurities to bedischarged from the member in the reaction chamber, so that it can beinhibited that the impurities are mixed into a formed film.

In addition, this invention is a film-forming unit that forms a thinfilm on an object to be processed by supplying a process gas into areaction chamber containing the object to be processed, the film-formingunit comprising: a nitrogen-including-gas supplying unit that suppliesinto the reaction chamber a nitrogen-including gas that includesnitrogen and that is capable of being activated; an activating unit thatactivates the nitrogen-including gas; and a nitriding unit that nitridesa surface of a member in the reaction chamber by controlling theactivating unit so as to activate the nitrogen-including gas.

According to the invention, a surface of a member in the reactionchamber is nitrided by the activated nitrogen-including gas. Thus, itbecomes difficult for impurities to be discharged from the member in thereaction chamber, so that it can be prevented that the impurities aremixed into a formed thin film.

Alternatively, this invention is a film-forming unit that forms a thinfilm on an object to be processed by supplying a process gas into areaction chamber containing the object to be processed, the film-formingunit comprising: a nitrogen-including-gas supplying unit that suppliesinto the reaction chamber a nitrogen-including gas that includesnitrogen and that is capable of being activated; an activating unit thatactivates the nitrogen-including gas; and a contaminant-removalcontrolling unit that removes metallic contaminant from a member in thereaction chamber by controlling the activating unit so as to activatethe nitrogen-including gas and by causing the activatednitrogen-including gas to react with the metallic contaminant containedin the member.

According to the feature, the nitrogen-including gas activated by theactivating unit reacts with the metallic contaminant contained in amember in the reaction chamber, and thus the metallic contaminant isremoved from the member. Therefore, an amount of metallic contaminantcontained in the member in the reaction chamber may be reduced, anddiffusion of the metallic contaminant during the film-forming processmay be inhibited. Thus, density of the metallic contaminant in a formedfilm may be reduced. In addition, it becomes difficult for impurities tobe mixed into a formed film.

Alternatively, this invention is a film-forming unit that forms a thinfilm on an object to be processed by supplying a process gas into areaction chamber containing the object to be processed, the film-formingunit comprising: a cleaning-gas supplying unit that supplies into thereaction chamber a cleaning gas that includes fluorine; anitrogen-including-gas supplying unit that supplies into the reactionchamber a nitrogen-including gas that includes nitrogen and that iscapable of being activated; an activating unit that activates thenitrogen-including gas; and a fluorine-removal controlling unit thatremoves fluorine from a member in the reaction chamber by controllingthe activating unit so as to activate the nitrogen-including gas and bycausing the activated nitrogen-including gas to react with the fluorinediffused into the member.

According to the feature, the nitrogen-including gas activated by theactivating unit reacts with the fluorine diffused into a member in thereaction chamber, and thus the fluorine is removed from the member.Therefore, an amount of fluorine diffused into the member in thereaction chamber may be reduced, and diffusion of the fluorine duringthe film-forming process may be inhibited. Thus, density of the fluorinein a formed film may be reduced. In addition, it becomes difficult forimpurities to be mixed into a formed film.

Alternatively, this invention is a film-forming unit that forms a thinfilm on an object to be processed by supplying a process gas into areaction chamber containing the object to be processed, the film-formingunit comprising: a cleaning-gas supplying unit that supplies into thereaction chamber a cleaning gas that includes fluorine; anitrogen-including-gas supplying unit that supplies into the reactionchamber a nitrogen-including gas that includes nitrogen and that iscapable of being activated; an activating unit that activates thenitrogen-including gas; and a nitriding unit that nitrides a surface ofa member in the reaction chamber by controlling the activating unit soas to activate the nitrogen-including gas.

According to the feature, a surface of a member in the reaction chamberis nitrided by the nitrogen-including gas activated by the activatingunit. Thus, it becomes difficult for the fluorine to diffuse (bedischarged) from the member in the reaction chamber, so that diffusionof the fluorine during the film-forming process may be inhibited. Thus,density of the fluorine in a formed film may be reduced. In addition, itcan be inhibited that impurities are mixed into a formed film.

The nitrogen-including gas is, for example, ammonia, dinitrogen monoxideor nitric oxide.

The activating unit is, for example, a heating unit. Alternatively, theactivating unit is a plasma-generating unit. Alternatively, theactivating unit is a photodecomposition unit. Alternatively, theactivating unit is a catalytic activating unit.

Preferably, the activating unit is a heating unit that heats the insideof the reaction chamber to a range of 600° C. to 1050° C.

In addition, preferably, the film-forming unit further comprises apressure-adjusting unit that maintains the inside of the reactionchamber at a range of 133 Pa to 53.3 kPa.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic longitudinal sectional view of a film-forming unitof an embodiment according to the invention;

FIG. 2 is a view showing a recipe for explaining a film-forming methodof an embodiment according to the invention;

FIG. 3 is a view showing a recipe for explaining a film-forming methodof another embodiment according to the invention;

FIG. 4 is a graph showing a relationship between depth of quartz chipand fluorine density;

FIG. 5 is a graph showing a relationship between depth of quartz chipand secondary ion strength of nitrogen;

FIG. 6 is a graph showing a relationship between purge gases and copperdensity;

FIG. 7 is a schematic longitudinal sectional view of a film-forming unitof another embodiment according to the invention; and

FIG. 8 is a schematic longitudinal sectional view of a conventionalfilm-forming unit.

DESCRIPTION OF THE PREFERRED EMBODIMENT

An embodiment of a cleaning method of a film-forming unit according tothe invention will now be described in detail with reference to abatch-type vertical thermal processing unit 1 shown in FIG. 1.

As shown in FIG. 1, the thermal processing unit 1 includes asubstantially cylindrical reaction tube 2 whose longitudinal axis isarranged in a vertical direction. The reaction tube 2 has a double-tubestructure consisting of an inner tube 3 and an outer tube 4 surroundingthe inner tube 3. A gap between the inner tube 3 and the outer tube 4 isconstant. Only the outer tube 4 has a ceiling. The inner tube 3 and theouter tube 4 are made of a heat-resistant material such as quartz.

A cylindrical manifold 5 made of a stainless steel (SUS) is arrangedbelow the outer tube 4. The manifold 5 is hermetically connected to alower end of the outer tube 4. The inner tube 3 is supported by asupporting ring 6, which projects from an inside wall of the manifold 5.

A lid 7 is arranged below the manifold 5. The lid 7 is verticallymovable by means of a boar elevator 8. When the lid 7 is moved up by theboat elevator 8, a lower end of the manifold 5 is closed.

A wafer boat 9 is placed on the lid 7. The wafer boat 9 is made of forexample quartz. The wafer boat 9 can contain a plurality of objects tobe processed such as semiconductor wafers 10 in a vertical tier-likemanner.

The reaction tube 2 is surrounded by a thermal insulation body 11.Heaters 12, each of which consists of for example a resistor heater, areprovided on an inside surface of the insulation body 11. The heaters 12heat the inside of the reaction tube 2 to a predetermined temperature,so that the semiconductor wafers 10 are heated to a predeterminedtemperature.

A plurality of process-gas-introducing tubes 13 for introducing aprocess gas are pierced through a side wall of the manifold 5. Only oneprocess-gas-introducing tube 13 is shown in FIG. 1 for simplification ofthe drawing. The plurality of process-gas-introducing tubes 13 areprovided below the supporting ring 6 and opened to the inside of theinner tube 3.

The plurality of process-gas-introducing tubes 13 are connected to apredetermined process-gas supplying source via mass flow controllers orthe like, not shown. If silicon nitride films (SiN films) are formed onthe semiconductor wafers 10, they are connected to an ammonia-gassupplying source and a silicon-including-gas supplying source. Thesilicon-including-gas is, for example, dichlorosilane (SiH₂Cl₂: DCS),hexachlorosilane (Si₂Cl₆), monosilane (SiH₄), disilane (Si₂H₆),tetrachlorosilane (SiCl₄), trichlorosilane (SiHCl₃),bis(tert-butylamino)silane or hexaethyl(amino)disilane. In the presentembodiment, they are connected to a DCS-gas supplying source. Thus, anammonia gas and a DCS gas are introduced into the inner tube 3 throughthe process-gas-introducing tubes 13 at predetermined flow rates.

A plurality of cleaning-gas-introducing tubes 14 for introducing acleaning gas are pierced through the side wall of the manifold 5. Onlyone cleaning-gas-introducing tube 14 is shown in FIG. 1 forsimplification of the drawing. The plurality of cleaning-gas-introducingtubes 14 are opened to the inside of the inner tube 3, so that thecleaning gas is adapted to be introduced into the inner tube 3 throughthe cleaning-gas-introducing tubes 14. In addition, thecleaning-gas-introducing tubes 14 are connected to a predeterminedcleaning-gas supplying source such as a fluorine-gas supplying source, ahydrogen-fluoride-gas supplying source and a nitrogen-gas supplyingsource, not shown, via mass flow controllers or the like, not shown.

A nitrogen-including-gas introducing tube 15 for introducing anitrogen-including gas is pierced through the side wall of the manifold5. The nitrogen-including gas includes nitrogen and is capable of beingactivated. For example, the nitrogen-including gas is ammonia,dinitrogen monoxide (N₂O) or nitric oxide (NO). The nitrogen-includinggas can nitride a member in the thermal processing unit 1, for example amember made of quartz.

The nitrogen-including-gas introducing tube 15 is opened to the insideof the inner tube 3. In addition, the nitrogen-including-gas introducingtube 15 is connected to a gas supplying source, not shown, via mass flowcontrollers or the like, not shown. Thus, the nitrogen-including gas isadapted to be introduced from the gas supplying source not shown intothe inner tube 3 through the nitrogen-including-gas introducing tube 15.

A discharging port 16 is also provided at the side wall of the manifold5. The discharging port 16 is located above the supporting ring 6 andcommunicates with a space (gap) defined between the inner tube 3 and theouter tube 4. Then, exhaust gas or the like generated in the inner tube3 is discharged into the discharging port 16 through the space betweenthe inner tube 3 and the outer tube 4. In addition, a purge-gassupplying tube 17 for supplying a nitrogen gas as a purge gas is piercedthrough the side wall of the manifold 5 below the discharging port 16.

The discharging port 16 is hermetically connected to a discharging duct18. In the discharging duct 18, a valve 19 and a vacuum pump 20 areprovided in turn from an upstream side (discharging port side) of thedischarging duct 18. An open level of the discharging duct 18 isadjusted by the valve 19. Thus, a pressure in the reaction tube 2 iscontrolled to a predetermined pressure. The vacuum pump 20 dischargesgas from an inside of the reaction tube 2 via the discharging duct 18and adjusts the pressure in the reaction tube 2.

In addition, in the discharging duct 18, a trap, a scrubber, and so on,not shown, are also provided. Thus, the exhaust gas discharged from thereaction tube 2 is made harmless and then discharged outside the thermalprocessing unit 1.

In addition, a controller 21 is connected to the boat elevator 8, theheater 12, the process-gas introducing tubes 13, the cleaning-gasintroducing tubes 14, the nitrogen-including-gas introducing tube 15,the purge-gas supplying tube 17, the valve 19 and the vacuum pump 20,respectively. The controller 21 may consist of a microprocessor, aprocess controller or the like. The controller 21 measures temperaturesand pressures at a plurality of positions of the thermal processing unit1, respectively. Then, the controller 21 outputs controlling signals orthe like to each of the above components based on the measured data, inorder to control the above components according to a recipe (timesequence) shown in FIG. 2 or 3.

Next, a cleaning method for the thermal processing unit 1 having theabove structure, and a film-forming method including the cleaning methodfor the thermal processing unit 1 are explained. In the presentembodiment, an ammonia gas and a DCS gas are introduced into thereaction tube 2 so as to form silicon nitride films on the semiconductorwafers 10. In the following explanation, the controller 21 controls eachof components constituting the thermal processing unit 1.

At first, with reference to the recipe shown in FIG. 2, a film-formingmethod including a purge process that is a cleaning method for thethermal processing unit 1, and a film-forming process for formingsilicon nitride films on the semiconductor wafers 10, is explained.

The heater 12 heats the inside of the reaction tube 2 to a predeterminedloading temperature, 300° C. in the present embodiment as shown in FIG.2 (a). As shown in FIG. 2 (c), a predetermined amount of nitrogen gas issupplied into the reaction tube 2 through the purge-gas supplying tube17, and then the wafer boat 9 not containing the semiconductor wafers 10is placed on the lid 7. Then, the lid 7 is moved up by the boat elevator8, and the reaction tube 2 is sealed (loading step).

Next, the gas in the reaction tube 2 is discharged, so that the insideof the reaction tube 2 is set at a predetermined pressure. The pressurein the reaction tube 2 is set at preferably 133 pa (1.0 Torr) to 53.3kPa (400 Torr). If the pressure is below 133 Pa (1.0 Torr), during anammonia-purging step described below, it is possible that outwarddiffusion of impurities (metallic contaminant, fluorine, and so on) inquartz forming the reaction tube 2 and nitridation of the quartz formingthe reaction tube 2 are inhibited. More preferably, the pressure in thereaction tube 2 is set at 2660 Pa (20 Torr) to 53.3 kPa (400 Torr). Ifthe pressure is above 2660 Pa (20 Torr), during the ammonia-purgingstep, the outward diffusion of the impurities and the nitridation of thequartz are promoted. In the present embodiment, as shown in FIG. 2 (b),the pressure is set at 2660 pa (20 Torr).

The inside of the reaction tube 2 is heated to a predeterminedtemperature by the heater 12. The temperature in the reaction tube 2 isset at preferably 600° C. to 1050° C. If the temperature is below 600°C., during the ammonia-purging step, it is possible that the outwarddiffusion of the impurities (metallic contaminant, fluorine, and so on)in the quartz forming the reaction tube 2 and the nitridation of thequartz forming the reaction tube 2 are inhibited. On the other hand, ifthe temperature is above 1050° C., the temperature is beyond a softeningpoint of the quartz forming the reaction tube 2. More preferably, thetemperature in the reaction tube 2 is set at 800° C. to 1050° C. If thetemperature is above 800° C., during the ammonia-purging step, theoutward diffusion of the impurities and the nitridation of the quartzare promoted. In the present embodiment, as shown in FIG. 2 (a), thetemperature in the reaction tube 2 is increased to 900° C. The abovepressure-reducing and heating operation is continued until the inside ofthe reaction tube 2 is stabled at a predetermined pressure and apredetermined temperature (stabling step).

When the inside of the reaction tube 2 is stabled at a predeterminedpressure and a predetermined temperature, the nitrogen-including gas isintroduced into the inner tube 3 through the nitrogen-including-gasintroducing tube 15 at a predetermined flow rate. For example, as shownin FIG. 2 (d), an ammonia gas is supplied at a flow rate of 1 liter/min.After a predetermined time has elapsed, the open degree of the valve 19is controlled, the vacuum pump 20 is operated, and the gas in thereaction tube 2 is discharged. Then, the supply of the ammonia gas andthe exhaust of the gas in the reaction tube 2 are repeated plural times(ammonia-purging step).

Herein, in the quartz forming the reaction tube 2 or the like,impurities such as metallic contaminant are included. It is difficult tomanufacture the reaction tube 2 without mixing impurities into thequartz forming the reaction tube 2 or the like. Specifically, a metalsuch as copper may be included depending on the manufacturing step, themanufacturing atmosphere, and so on. If the ammonia gas is supplied intothe inner tube 3, the ammonia gas is activated by the heat in thereaction tube 2, and then reacts with the metallic contaminant containedin the quartz forming the reaction tube 2. Thus, it becomes easy for themetallic contaminant to diffuse (outward diffuse) from the quartzforming the reaction tube 2. Thus, the metallic contaminant contained inthe quartz forming the reaction tube 2 is reduced, so that diffusion ofthe metallic contaminant from the reaction tube 2 can be reduced duringthe film-forming process. As a result, an amount (density) of metalliccontaminant contained in the silicon nitride films formed by thefilm-forming process can be reduced.

In addition, in the quartz forming the reaction tube 2 or the like,fluorine may be mixed (diffused) by a cleaning process described below.In the case, when the ammonia gas is supplied into the inner tube 3, theactivated ammonia gas reacts with the fluorine that has been diffusedinto the quartz, and hence the fluorine may easily diffuse (outwarddiffuse) from the quartz of the reaction tube 2. Thus, the fluorinediffused into the quartz forming the reaction tube 2 is reduced, so thatdiffusion of the fluorine from the reaction tube 2 can be reduced duringthe film-forming process. As a result, an amount (density) of fluorinecontained in the silicon nitride films formed by the film-formingprocess can be reduced. In addition, it can be prevented that fluorineimpurities are mixed into the silicon nitride films.

Furthermore, the activated ammonia gas nitrides a surface of the quartzforming the reaction tube 2 or the like. This makes it difficult for theimpurities to outward diffuse from the quartz into the reaction tube 2,so that it can be prevented that the impurities such as the metalliccontaminant are mixed into the silicon nitride films formed by thefilm-forming process. In particular, when a nitride film is formed bynitriding a surface of the quartz forming the reaction tube 2 or thelike by using radicals such as N* and NH* of the ammonia gas, it becomesdifficult for the impurities such as the metallic contaminant to bedischarged from the quartz into the reaction tube 2. Thus, it is morepreferable to form a nitride film on a surface of the quartz forming thereaction tube 2 or the like by the activated ammonia gas.

Next, the open degree of the valve 19 is controlled, the vacuum pump 20is operated, and the gas in the reaction tube 2 is discharged. On theother hand, as shown in FIG. 2 (c), a predetermined amount of nitrogengas is supplied from the purge-gas supplying tube 17. The gas in thereaction tube 2 is discharged to the discharging duct 18. In addition,the heater 12 adjusts the inside of the reaction tube 2 at apredetermined temperature, for example 300° C. as shown in FIG. 2(a).Then, as shown in FIG. 2 (b), the pressure in the reaction tube 2 isreturned back to a normal pressure (stabling step). Then, the lid 7 ismoved down by the boat elevator 8 and unloaded (unloading step).

After the thermal processing unit 1 is cleaned as described above, afilm-forming process that forms silicon nitride films on thesemiconductor wafers 10 is carried out.

At first, the heater 12 heats the inside of the reaction tube 2 at apredetermined loading temperature, for example 300° C. as shown in FIG.2(a). In addition, in a state wherein the lid 7 is located at a lowerposition by the boat elevator 8, the wafer boat 9 containing thesemiconductor wafers 10 is placed on the lid 7. Then, as shown in FIG. 2(c), a predetermined amount of nitrogen gas is supplied from thepurge-gas supplying tube 17 into the reaction tube 2. Then, the lid 7 ismoved up by the boat elevator 8, and the wafer boat 9 is loaded into thereaction tube 2. Thus, the semiconductor wafers 10 are contained in theinner tube 3 of the reaction tube 2, and the reaction tube 2 ishermetically closed (loading step).

After the reaction tube 2 is hermetically closed, the open level of thevalve 19 is controlled and the vacuum pump 20 is operated. Thus, the gasin the reaction tube 2 is discharged and the pressure in the reactiontube 2 is decompressed to a predetermined pressure, for example 26.5 Pa(0.2 Torr) as shown in FIG. 2(b). In addition, the heater 12 heats theinside of the reaction tube 2 to a predetermined temperature, forexample 760° C. as shown in FIG. 2(a). The above pressure-reducing andheating operation is continued until the inside of the reaction tube 2is stabled at a predetermined pressure and a predetermined temperature(stabling step).

When the inside of the reaction tube 2 is stabled at a predeterminedpressure and a predetermined temperature, the supply of the nitrogen gasfrom the purge-gas supplying tube 17 is stopped. Then, the ammonia gasas a process gas is supplied from the process-gas introducing tubes 13into the inner tube 3, for example at a flow rate of 0.75 liter/min asshown in FIG. 2 (d), and the DCS gas as a process gas is also suppliedfrom the process-gas introducing tubes 13 into the inner tube 3, forexample at a flow rate of 0.075 liter/min as shown in FIG. 2 (e).

When the ammonia gas and the DCS gas are introduced, a thermaldecomposition reaction is caused by the heat in the reaction tube 2, sothat silicon nitride is deposited on surfaces of the semiconductorwafers 10. Thus, silicon nitride films are formed on the surfaces of thesemiconductor wafers 10 (film-forming step).

When silicon nitride films having a predetermined thickness are formedon the surfaces of the semiconductor wafers 10, the supply of theammonia gas and the DCS gas from the process-gas introducing tubes 13 isstopped. Then, the open level of the valve 19 is controlled, the vacuumpump 20 is operated, and the gas in the reaction tube 2 is discharged.On the other hand, as shown in FIG. 2 (c), a predetermined amount ofnitrogen gas is supplied from the purge-gas supplying tube 17. The gasin the reaction tube 2 is discharged to the discharging duct 18 (purgingstep). In order to surely discharge the gas in the reaction tube 2, itis preferable to repeat the gas-discharging step of the reaction tube 2and the supplying step of the nitrogen gas plural times.

Finally, as shown in FIG. 2 (c), a predetermined amount of the nitrogengas is supplied through the purge-gas supplying tube 17, and thepressure in the reaction tube 2 is returned back to a normal pressure.Then, the lid 7 is moved down by the boat elevator 8 so that the waferboat 9 (semiconductor wafers 10) is unloaded from the reaction tube 2(unloading step).

The above film-forming process may be repeated plural times after thepurging process. For example, after the thermal processing unit 1 iscleaned by the purging process, the film-forming process may be repeateda predetermined number of times. Thus, the silicon nitride films can beformed on the semiconductor wafers 10 continuously. In addition, whenthe purging process and the film-forming process are always alternatelyconducted, mixing of the metallic contaminant and the fluorine into theformed silicon nitride films can be reduced.

According to the above film-forming method, the amount of the metalliccontaminant and/or the fluorine in the quartz forming the reaction tube2 can be reduced, so that the diffusion of the metallic contaminant orthe like from the reaction tube 2 during the film-forming process can bereduced. As a result, the mixing of the impurities into the siliconnitride films formed by the film-forming process can be reduced, so thatthe density of the impurities in the silicon nitride films can bereduced.

In addition, when a nitride film is formed by nitriding a surface of thequartz forming the reaction tube 2 or the like by using radicals such asN* and NH* of the activated ammonia gas, it becomes more difficult forthe impurities to diffuse (outward diffuse) from the quartz into thereaction tube 2. As a result, the mixing of the impurities into thesilicon nitride films formed by the film-forming process can be reduced,so that the density of the impurities in the silicon nitride films canbe reduced.

Next, with reference to a recipe shown in FIG. 3, a film-forming methodincluding a film-forming process, a cleaning process for removing thesilicon nitride stuck to the inner surfaces of the thermal processingunit 1, and a purging process is explained. The cleaning process and thepurging process correspond to a cleaning method for a film-forming unitaccording to the invention.

At first, the heater 12 heats the inside of the reaction tube 2 at apredetermined loading temperature, for example 300° C. as shown in FIG.3 (a). In addition, in a state wherein the lid 7 is located at a lowerposition by the boat elevator 8, the wafer boat 9 containing thesemiconductor wafers 10 is placed on the lid 7. Then, as shown in FIG. 3(c), a predetermined amount of nitrogen gas is supplied from thepurge-gas supplying tube 17 into the reaction tube 2. Then, the lid 7 ismoved up by the boat elevator 8, and the wafer boat 9 is loaded into thereaction tube 2. Thus, the semiconductor wafers 10 are contained in theinner tube 3 of the reaction tube 2, and the reaction tube 2 ishermetically closed (loading step).

After the reaction tube 2 is hermetically closed, the open level of thevalve 19 is controlled and the vacuum pump 20 is operated. Thus, the gasin the reaction tube 2 is discharged and the pressure in the reactiontube 2 is decompressed to a predetermined pressure, for example 26.5 Pa(0.2 Torr) as shown in FIG. 3 (b). In addition, the heater 12 heats theinside of the reaction tube 2 to a predetermined temperature, forexample 760° C. as shown in FIG. 3 (a). The above pressure-reducing andheating operation is continued until the inside of the reaction tube 2is stabled at a predetermined pressure and a predetermined temperature(stabling step).

When the inside of the reaction tube 2 is stabled at a predeterminedpressure and a predetermined temperature, the supply of the nitrogen gasfrom the purge-gas supplying tube 17 is stopped. Then, the ammonia gasas a process gas is supplied from the process-gas introducing tubes 13into the inner tube 3, for example at a flow rate of 0.75 liter/min asshown in FIG. 3 (d), and the DCS gas as a process gas is also suppliedfrom the process-gas introducing tubes 13 into the inner tube 3, forexample at a flow rate of 0.075 liter/min as shown in FIG. 3 (e).

When the ammonia gas and the DCS gas are introduced, a thermaldecomposition reaction is caused by the heat in the reaction tube 2, sothat silicon nitride is deposited on surfaces of the semiconductorwafers 10. Thus, silicon nitride films are formed on the surfaces of thesemiconductor wafers 10 (film-forming step).

When silicon nitride films having a predetermined thickness are formedon the surfaces of the semiconductor wafers 10, the supply of theammonia gas and the DCS gas from the process-gas introducing tubes 13 isstopped. Then, the open level of the valve 19 is controlled, the vacuumpump 20 is operated, and the gas in the reaction tube 2 is discharged.On the other hand, as shown in FIG. 3 (c), a predetermined amount ofnitrogen gas is supplied from the purge-gas supplying tube 17. The gasin the reaction tube 2 is discharged to the discharging duct 18 (purgingstep).

Finally, as shown in FIG. 3 (c), a predetermined amount of the nitrogengas is supplied through the purge-gas supplying tube 17, and thepressure in the reaction tube 2 is returned back to a normal pressure.Then, the lid 7 is moved down by the boat elevator 8 so that the waferboat 9 (semiconductor wafers 10) is unloaded from the reaction tube 2(unloading step).

After the above film-forming process is conducted plural times, thesilicon nitride formed during the film-forming process may be depositedon (stuck to) not only the surfaces of the semiconductor wafers 10, butalso the inner surfaces of the thermal processing unit 1 (film-formingunit) such as the inner wall of the inner tube 3. Thus, after thefilm-forming process is conducted a predetermined number of times, acleaning process that removes the silicon nitride stuck to the inside ofthe thermal processing unit 1 is conducted. During the cleaning process,a gas consisting of: a cleaning gas including a fluorine gas (F₂) suchas a fluorine gas itself, a hydrogen fluoride gas (HF), and a nitrogengas (N₂) as a diluent is supplied into the thermal processing unit 1(reaction tube 2). The cleaning process of the thermal processing unit 1is explained as follows.

At first, as shown in FIG. 3 (c), a predetermined amount of nitrogen gasis supplied into the reaction tube 2 through the purge-gas supplyingtube 17, and then the wafer boat 9 not containing the semiconductorwafers 10 is placed on the lid 7. Then, the lid 7 is moved up by theboat elevator 8, and the reaction tube 2 is sealed (loading step).

Next, the gas in the reaction tube 2 is discharged, so that the insideof the reaction tube 2 is maintained at a predetermined pressure, forexample 20000 Pa (150 Torr) as shown in FIG. 3 (b). In addition, theheater 12 heats (maintains) the inside of the reaction tube 2 at apredetermined temperature, for example 300° C. as shown in FIG. 3 (a).The above pressure-reducing and heating operation is continued until theinside of the reaction tube 2 is stabled at a predetermined pressure anda predetermined temperature (stabling step).

When the inside of the reaction tube 2 is stabled at a predeterminedpressure and a predetermined temperature, the cleaning gas is introducedinto the inner tube 3 through the cleaning-gas introducing tubes 14 at apredetermined flow rate. For example, a fluorine gas is supplied at aflow rate of 2 liter/min as shown in FIG. 3 (f), a hydrogen-fluoride gasis supplied at a flow rate of 2 liter/min as shown in FIG. 3 (g), and anitrogen gas is supplied at a flow rate of 8 liter/min as shown in FIG.3 (c). The introduced cleaning gas is heated in the inner tube 3, and isdischarged from the inner tube 3 to the discharging duct 18 through thespace formed between the inner tube 3 and the outer tube 4. During thatdischarge, the cleaning gas comes in contact with the silicon nitridestuck to the inner surfaces of the thermal processing unit 1, such asthe inner wall and the outer wall of the inner tube 3, the inner wall ofthe outer tube 4, the inner wall of the discharging duct 18, and thewafer boat 9, in order to etch the silicon nitride. Thus, the siliconnitride stuck to the inner surfaces of the thermal processing unit 1 isremoved (cleaning step).

Herein, when the fluorine gas is supplied into the reaction tube 2during the cleaning step, the fluorine gas may diffuse into the quartzforming the reaction tube 2, for example. If a film-forming process isconducted under a state wherein the fluorine has been diffused into thequartz of the reaction tube 2, the fluorine may diffuse (outwarddiffuse) from the reaction tube 2 during the film-forming process, sothat fluorine density in the silicon nitride film formed on thesemiconductor wafers 10 may be increased. In addition, as the fluorinediffuses outward from the reaction tube 2, it is possible that fluorineimpurities (for example, SiF) are mixed into the thin films formed onthe semiconductor wafers 10. Thus, after the cleaning process isconducted, a purging process that purges the inside of the thermalprocessing unit 1 is conducted. The purging process is explained asfollows.

At first, the supply of the cleaning gas from the cleaning-gas supplyingtubes 14 is stopped. Then, a predetermined amount of nitrogen gas issupplied from the purge-gas supplying tube 17, and the gas in thereaction tube 2 is discharged. On the other hand, the pressure in thereaction tube 2 is set at a predetermined pressure, for example 133 pa(1.0 Torr) to 53.3 kPa (400 Torr) as described above. In the presentembodiment, the pressure is set at 2660 Pa (20 Torr), as shown in FIG. 3(b). In addition, the inside of the reaction tube 2 is set at apredetermined temperature, for example 600° C. to 1050° C. as describedabove, by the heater 12. In the present embodiment, the temperature isincreased to 900° C., as shown in FIG. 3 (a). The abovepressure-reducing and heating operation is continued until the inside ofthe reaction tube 2 is stabled at a predetermined pressure and apredetermined temperature (stabling step).

When the inside of the reaction tube 2 is stabled at a predeterminedpressure and a predetermined temperature, the nitrogen-including gas isintroduced into the inner tube 3 through the nitrogen-including-gasintroducing tube 15 at a predetermined flow rate. For example, as shownin FIG. 3 (d), an ammonia gas is supplied at a flow rate of 1 liter/min.After a predetermined time has elapsed, the open degree of the valve 19is controlled, the vacuum pump 20 is operated, and the gas in thereaction tube 2 is discharged. Then, the supply of the ammonia gas andthe exhaust of the gas in the reaction tube 2 are repeated plural times(ammonia-purging step).

When the ammonia gas is supplied into the inner tube 3, the ammonia gasis activated (excited) by the heat in the reaction tube 2. The activatedammonia easily reacts with the fluorine that has been diffused into thequartz forming the reaction tune 2, in order to generate ammoniumfluoride (NH₄F), for exampl. Thus, the fluorine is discharged out fromthe reaction tube 2. Thus, an amount of the fluorine that has beendiffuse into the quartz forming the reaction tube 2 is reduced, so thatdiffusion of the fluorine from the reaction tube 2 during thefilm-forming process can be reduced. As a result, fluorine density inthe silicon nitride film formed by the film-forming process can bereduced. In addition, it can be inhibited that fluorine impurities suchas SiF are mixed into the silicon nitride film.

In addition, the activated ammonia may react with metallic contaminantcontained in the quartz forming the reaction tube 2. Thus, it becomeseasier for the metallic contaminant to diffuse (outward diffuse) fromthe quarts of the reaction tube 2. Thus, the metallic contaminantcontained in the quartz forming the reaction tube 2 is reduced, so thatdiffusion of the metallic contaminant from the reaction tube 2 duringthe film-forming process can be reduced. As a result, an amount(density) of the metallic contaminant in the silicon nitride film formedby the film-forming process can be reduced.

In addition, the activated ammonia may nitride a surface of the quartzforming the reaction tube 2 or the like. Thus, it becomes difficult forthe fluorine in the quartz to diffuse from the reaction tube 2, so thatthe diffusion of the fluorine from the reaction tube 2 during thefilm-forming process can be reduced. As a result, fluorine density inthe silicon nitride film formed by the film-forming process can bereduced. In addition, it can be inhibited that impurities are mixed intothe silicon nitride film. In particular, when a nitride film is formedby nitriding a surface of the quartz forming the reaction tube 2 or thelike by using radicals such as N* and NH* of the ammonia gas, it becomesdifficult for the impurities to diffuse from the quartz into thereaction tube 2. Thus, it is more preferable to form a nitride film on asurface of the quartz forming the reaction tube 2 or the like by theactivated ammonia gas.

Next, the open degree of the valve 19 is controlled, the vacuum pump 20is operated, and the gas in the reaction tube 2 is discharged. On theother hand, a predetermined amount of nitrogen gas is supplied from thepurge-gas supplying tube 17. The gas in the reaction tube 2 isdischarged to the discharging duct 18. In addition, the heater 12adjusts the inside of the reaction tube 2 at a predeterminedtemperature, for example 300° C. as shown in FIG. 3 (a). Then, as shownin FIG. 3 (b), the pressure in the reaction tube 2 is returned back to anormal pressure (stabling step). Then, the lid 7 is moved down by theboat elevator 8 and unloaded (unloading step). Then, the wafer boat 9containing the semiconductor wafers 10 is placed on the lid 7. Thus, afilm-forming process for forming silicon nitride films on thesemiconductor wafers 10 may be conducted.

As described above, by repeating the cleaning method for thefilm-forming unit including the cleaning process and the purging processafter a predetermined times of the film-forming processes, siliconnitride films can be formed on the semiconductor wafers 10 continuously.Herein, after each film-forming process, the cleaning process and thepurging process may be conducted. In the case, the inside of the furnace(the inside of the reaction tube 2) is cleaned each time, so that mixingof the metallic contaminant and/or the fluorine into the formed siliconnitride films may be reduced.

In the above film-forming method, the amount of fluorine, which has beendiffused into the quartz forming the reaction tube 2 during the cleaningprocess, can be reduced, so that the diffusion of the fluorine or thelike from the reaction tube 2 during the film-forming process can bereduced. Thus, the fluorine density in the silicon nitride film formedby the film-forming process can be reduced. In addition, it can beinhibited that fluorine impurities such as SiF are mixed into thesilicon nitride film. That is, the mixing of the impurities into thesilicon nitride films formed by the film-forming process can be reduced,so that the density of the impurities in the silicon nitride films canbe reduced.

In addition, when a nitride film is formed by nitriding a surface of thequartz forming the reaction tube 2 or the like by using radicals such asN* and NH* of the activated ammonia gas, it becomes more difficult forthe impurities to diffuse (outward diffuse) from the quartz into thereaction tube 2. As a result, the mixing of the impurities into thesilicon nitride films formed by the film-forming process can be reduced,so that the density of the impurities in the silicon nitride films canbe reduced.

Next, in order to confirm an effect of the present embodiment, after aquartz chip was contained in the thermal processing unit 1 (reactiontube 2) and a cleaning process using a cleaning gas including a fluorinegas was conducted, a conventional nitrogen-purging (N₂ purge) using anitrogen gas was conducted or an ammonia-purging (NH₃ purge) using anammonia gas according to the invention was conducted, and then fluorinedensity in a depth direction of the quartz chip was measured. Inaddition, secondary ion strength of nitrogen was measured by a secondaryion mass spectrometry (SIMS).

Herein, the cleaning process and the ammonia-purging were conducted inaccordance with the above embodiment. The nitrogen-purging was conductedunder the same conditions as the ammonia-purging except that thenitrogen gas was used as a purge gas. FIG. 4 shows a relationshipbetween depth of quartz chip and fluorine density. FIG. 5 shows arelationship between depth of quartz chip and secondary ion strength ofnitrogen.

As shown in FIG. 4, it was confirmed that the amount of fluorinediffused into the quartz chip may be reduced (inhibited) by conductingthe ammonia-purging. In particular, it was confirmed that the amount offluorine may be greatly reduced (inhibited) in the vicinity of thesurface of the quartz chip. The reason may be thought because theactivated ammonia reacts with the fluorine diffused in the vicinity ofthe surface of the quartz chip and the fluorine is discharged.

In addition, as shown in FIG. 5, it was confirmed that the secondary ionstrength of nitrogen may be enhanced by conducting the ammonia-purging.In particular, it was confirmed that the secondary ion strength ofnitrogen may be greatly enhanced in the vicinity of the surface of thequartz chip. That is, the vicinity of the surface of the quartz chip isnitrided by the ammonia-purging.

Next, in order to confirm an effect of the present embodiment, after afilm-forming process and a cleaning process were conducted, wafers wereloaded into the reaction tube 2 that has been subjected to aconventional nitrogen-purging (N₂ purge) using a nitrogen gas or anammonia-purging (NH₃ purge) using an ammonia gas according to theinvention, the inside of the reaction tube 2 was heated to 800° C. so asto heat the wafers, the heated wafers were taken out, and copper densityon a wafer surface was measured. The result is shown in FIG. 6. As shownin FIG. 6, the measurement of the copper density was conducted for fivepredetermined points on the wafer surface in accordance with a totalreflection X-ray fluorescence analyzing method. In addition, in theammonia-purging step, the temperature in the reaction tube 2 was 950°C., the pressure therein was 15960 Pa (120 Torr), and the ammonia gaswas supplied into the reaction tube 2 at a flow rate of 2 liter/minunder the above temperature and the above pressure.

As shown in FIG. 6, it was confirmed that the copper density on thewafer surface may be reduced to 1/10 by conducting the ammonia-purging.The reason may be thought because the activated ammonia reacts with thecopper existing in the quartz (reaction tube 2, wafer boat 9, or thelike) so as to discharge the copper from the quartz. Thus, it becomesdifficult for the copper to be discharged from the quartz during thefilm-forming process, so that diffusion of the copper during thefilm-forming process can be inhibited. In addition, the same densitymeasurements for chrome (Cr) and nickel (Ni) were conducted, and thus itwas confirmed that chrome density and nickel density in the siliconnitride film may be reduced by conducting the ammonia-purging.

As described above, according to the embodiment, since the amounts ofthe fluorine and the metallic contaminant in the reaction tube 2 may bereduced by the ammonia-purging, the diffusion of the fluorine and themetallic contaminant from the reaction tube 2 during the film-formingprocess may be reduced. As a result, fluorine density in the siliconnitride film formed by the film-forming process may be reduced. Inaddition, it can be inhibited that the impurities such as the metalliccontaminant are mixed into the silicon nitride film.

In addition, according to the embodiment, since the surface of thequartz forming the reaction tube 2 is nitrided by the ammonia-purging,the diffusion of the fluorine and the metallic contaminant from thereaction tube 2 during the film-forming process can be reduced. As aresult, fluorine density in the silicon nitride film formed by thefilm-forming process may be reduced. In addition, it can be inhibitedthat the impurities such as the metallic contaminant are mixed into thesilicon nitride film.

In addition, the invention is not limited to the above embodiment, butmay be variously modified and developed.

In the above embodiment, a nitrogen-including gas not activated issupplied into the reaction tube 2 heated to a predetermined temperature(900° C.) to be activated. However, for example as shown in FIG. 7, anactivating unit 31 may be provided in the nitrogen-including gasintroducing tube 15, and a nitrogen-including gas that has beenactivated may be supplied into the reaction tube 2. In the case, even ifthe temperature in the reaction tube 2 is below 600° C. during theammonia-purging step, outward diffusion of the impurities in the quartzand nitridation of the quartz may be satisfactorily conducted. That is,lowering of the temperature of the ammonia purging may be achieved. Asan activating unit 31, a heating unit, a plasma-generating unit, aphotodecomposition unit, a catalytic activating unit and so on may beused.

In the above embodiment, the ammonia gas is used as a nitrogen-includinggas. However, the nitrogen-including gas may be any gas that includesnitrogen and that is capable of being activated. For example, thenitrogen-including gas may be dinitrogen monoxide or nitric oxide. Inaddition, the cleaning gas may be any gas that includes fluorine. Forexample, the cleaning gas may consist of a gas including fluorine andchlorine such as ClF₃.

In the above embodiment, the reaction tube 2 or the like is made ofquartz. However, the material forming the reaction tube 2 or the like isnot limited to quartz. For example, the invention is effective for anymaterial into which fluorine can diffuse, such as any SiC material.Herein, since it is requested that the reaction tube 2 or the like hasgood heat resistance, it is preferable that the material is superior inheat resistance.

In the above embodiment, the silicon nitride films are formed on thesemiconductor wafers 10. However, this invention is also effective for afilm-forming unit that forms titanium nitride films on the semiconductorwafers 10.

In the above embodiment, the ammonia purge is conducted under thecondition wherein the temperature in the reaction tube 2 is set at 900°C. and the pressure therein is set at 2660 Pa (20 Torr). However, thetemperature and the pressure in the reaction tube 2 are not limitedthereto. For example, the temperature in the reaction tube 2 may be setat 950° C. and the pressure therein may be set at 15960 Pa (120 Torr).If the temperature and the pressure in the reaction tube 2 are increasedlike this, the surface of the quartz of the reaction tube 2 is nitridedmore, so that the diffusion of the fluorine or the like from thereaction tube 2 during the film-forming process can be inhibited more.In addition, frequency of the cleaning process may be one time forseveral film-forming processes or one time for each film-formingprocess.

In the above embodiment, the batch-type of vertical thermal processingunit having a double-tube structure is explained wherein the reactiontube 2 is formed by the inner tube 3 and the outer tube 4. However, theinvention is not limited thereto. For example, the invention isallocable to any batch-type of thermal processing unit having asingle-tube structure not including the inner tube 3. In addition, theobject to be processed is not limited to the semiconductor wafer 10, butmay be a glass substrate for an LCD.

1. A cleaning method of a film-forming unit that forms a thin film on anobject to be processed by supplying a process gas into a reactionchamber containing the object to be processed, the method comprising; apurging step of purging an inside of the reaction chamber by supplyinginto the reaction chamber a nitrogen-including gas that includesnitrogen and that is capable of being activated, wherein the purgingstep has a step of nitriding a surface of a member in the reactionchamber by activating the nitrogen-including gas.
 2. A cleaning methodof a film-forming unit that forms a thin film on an object to beprocessed by supplying a process gas into a reaction chamber containingthe object to be processed, the method comprising; a purging step ofpurging an inside of the reaction chamber by supplying into the reactionchamber a nitrogen-including gas that includes nitrogen and that iscapable of being activated, wherein the purging step has a step ofactivating the nitrogen-including gas and causing the activatednitrogen-including gas to react with metallic contaminant contained in amember in the reaction chamber so as to remove the metallic contaminantfrom the member.
 3. A cleaning method of a film-forming unit that formsa thin film on an object to be processed by supplying a process gas intoa reaction chamber containing the object to be processed, the methodcomprising; a deposit-removing step of removing a deposit stuck to aninside of the film-forming unit by supplying into the reaction chamber acleaning gas that includes fluorine, and a purging step of purging aninside of the reaction chamber by supplying into the reaction chamber anitrogen-including gas that includes nitrogen and that is capable ofbeing activated, wherein the purging step has a step of activating thenitrogen-including gas and causing the activated nitrogen-including gasto react with the fluorine diffused into a member in the reactionchamber during the deposit-removing step, so as to remove the fluorinefrom the member.
 4. A cleaning method of a film-forming unit that formsa thin film on an object to be processed by supplying a process gas intoa reaction chamber containing the object to be processed, the methodcomprising; a deposit-removing step of removing a deposit stuck to aninside of the film-forming unit by supplying into the reaction chamber acleaning gas that includes fluorine, and a purging step of purging aninside of the reaction chamber by supplying into the reaction chamber anitrogen-including gas that includes nitrogen and that is capable ofbeing activated, wherein the purging step has a step of nitriding asurface of a member in the reaction chamber by activating thenitrogen-including gas.
 5. A cleaning method of a film-forming unitaccording to any of claims 1 to 4, wherein the nitrogen-including gas isammonia, dinitrogen monoxide or nitric oxide.
 6. A cleaning method of afilm-forming unit according to any of claims 1 to 4, wherein during thepurging step, the inside of the reaction chamber is maintained at arange of 133 Pa to 53.3 kPa.
 7. A cleaning method of a film-forming unitaccording to any of claims 1 to 4, wherein during the purging step, thenitrogen-including gas is supplied into the reaction chamber heated to apredetermined temperature in order to be activated.
 8. A cleaning methodof a film-forming unit according to claim 7, wherein during the purgingstep, the inside of the reaction chamber is heated to a range of 600° C.to 1050° C.
 9. A cleaning method of a film-forming unit according to anyof claims 1 to 4, wherein the member in the reaction chamber consists ofquartz.
 10. A cleaning method of a film-forming unit according to any ofclaims 1 to 4, wherein the process gas comprises ammonia and asilicon-including gas, the thin film is a silicon nitride film, and thenitrogen-including gas is an ammonia gas.
 11. A film-forming methodcomprising a cleaning step of cleaning a film-forming unit in accordancewith a cleaning method of a film-forming unit according to any of claims1 to 4, and a film-forming step of heating the inside of the reactionchamber containing the object to be processed to a predeterminedtemperature, and forming a thin film on the object to be processed bysupplying a process gas into the reaction chamber.
 12. A film-formingunit that forms a thin film on an object to be processed by supplying aprocess gas into a reaction chamber containing the object to beprocessed, the film-forming unit comprising; a nitrogen-including-gassupplying unit that supplies into the reaction chamber anitrogen-including gas that includes nitrogen and that is capable ofbeing activated, an activating unit that activates thenitrogen-including gas, and a nitriding unit that nitrides a surface ofa member in the reaction chamber by controlling the activating unit soas to activate the nitrogen-including gas.
 13. A film-forming unit thatforms a thin film on an object to be processed by supplying a processgas into a reaction chamber containing the object to be processed, thefilm-forming unit comprising; a nitrogen-including-gas supplying unitthat supplies into the reaction chamber a nitrogen-including gas thatincludes nitrogen and that is capable of being activated, an activatingunit that activates the nitrogen-including gas, and acontaminant-removal controlling unit that removes metallic contaminantfrom a member in the reaction chamber by controlling the activating unitso as to activate the nitrogen-including gas and by causing theactivated nitrogen-including gas to react with the metallic contaminantcontained in the member.
 14. A film-forming unit that forms a thin filmon an object to be processed by supplying a process gas into a reactionchamber containing the object to be processed, the film-forming unitcomprising; a cleaning-gas supplying unit that supplies into thereaction chamber a cleaning gas that includes fluorine, anitrogen-including-gas supplying unit that supplies into the reactionchamber a nitrogen-including gas that includes nitrogen and that iscapable of being activated, an activating unit that activates thenitrogen-including gas, and a fluorine-removal controlling unit thatremoves fluorine from a member in the reaction chamber by controllingthe activating unit so as to activate the nitrogen-including gas and bycausing the activated nitrogen-including gas to react with the fluorinediffused into the member.
 15. A film-forming unit that forms a thin filmon an object to be processed by supplying a process gas into a reactionchamber containing the object to be processed, the film-forming unitcomprising; a cleaning-gas supplying unit that supplies into thereaction chamber a cleaning gas that includes fluorine, anitrogen-including-gas supplying unit that supplies into the reactionchamber a nitrogen-including gas that includes nitrogen and that iscapable of being activated, an activating unit that activates thenitrogen-including gas, and a nitriding unit that nitrides a surface ofa member in the reaction chamber by controlling the activating unit soas to activate the nitrogen-including gas.
 16. A film-forming unitaccording to any of claims 12 to 15, wherein the nitrogen-including gasis ammonia, dinitrogen monoxide or nitric oxide.
 17. A film-forming unitaccording to any of claims 12 to 15, wherein the activating unit is aheating unit.
 18. A film-forming unit according to any of claims 12 to15, wherein the activating unit is a plasma-generating unit.
 19. Afilm-forming unit according to any of claims 12 to 15, wherein theactivating unit is a heating unit that heats the inside of the reactionchamber to a range of 600° C. to 1050° C.
 20. A film-forming unitaccording to any of claims 12 to 15, further comprising apressure-adjusting unit that maintains the inside of the reactionchamber at a range of 133 Pa to 53.3 kPa.